The invention relates to laser driver systems, and relates in particular to laser driver systems that control average power and extinction ratio, or optical modulation amplitude.
Laser drivers are electronic circuits that provide a current that is used to power a laser, i.e., to cause it to emit light. The current source may be modulated (e.g., switched between two levels) to provide modulated information such as serial digital data. Additional current sources may be used to provide current for the purpose of controlling the average optical power of the laser. For example, the laser may be operated such that a digital zero is provided at a first optical output power level that is just above the lasing threshold of the laser. The laser may also be operated such that a digital one is provided at a second optical output power level that is above a higher threshold. Laser drivers facilitate the driving of such lasers (e.g., laser diodes) for purposes of data transmission, for example in a communication system, a printer, or a laser drilling system.
Laser Driver IC's are also used to control the average output power of the laser. The average power is the effective power with the laser switching on and off (high and low) at, for example, a 50% duty cycle. Average laser power is often calculated by adding the laser power for an optical one (called the P1 level) to the power level for an optical zero (called the P0 level), and multiplying the sum by the duty cycle. Typically, average power of the laser is measured by using a photodiode placed close to the back facet of the laser. The frequency response of the photodiode is much lower than the modulation rate of the laser. Being slower than the modulation rate, it averages the signal to provide the average power level and thus it appears as a DC signal.
For example, FIG. 1 shows a prior art laser driver that includes an output laser diode 10 that provides a laser output signal 12 as well as optical feedback 14 to a back-facet photo monitor diode 16. The photo monitor diode 16 is coupled to Vcc as shown and to an automatic power control loop (APC) 18. The APC 18 measures the current from the diode and compares it to the average power (PAV) set point signal 22. If there is a delta between those points, the APC 18 changes the bias current at the bias current source 20, which changes the draw of current through the laser diode 10. The APC tells the current source to increase or decrease. The current monitor is a signal 24 that indicates the amount of current in the current source. The bias current source 20, therefore, ensures that the laser is operating above its low threshold point, i.e., that there is always some amount of current flowing in the laser. The laser should have at least the minimum amount of current through it that it produces light: this is the P0 point. The bias current source also provides a bias current monitor signal 24 that is indicative of the current in the bias current source 20. Note that in the absence of a modulation current signal the bias current will rise until the laser matches the average power represented by the set-point. When a modulation current (Imod) is present, the bias current (Ibias) will decrease to account for the average power made by the modulation current. The bias current may decrease until it matches the threshold current, or P0 values of the laser.
The laser value of the modulation current will control the difference between the P1 and P0 level. The optical power is typically measure two ways. The first is called Optical Modulation Amplitude (OMA) and it simply P1-P0 and is typically measured in Watts of light energy. The second is know as Extinction Ratio (ER) defined as 10 LOG(P1/P0) and is in units of dB. While there are two ways to measure this, the result is the same. It is a measurement of the difference between an optical one and an optical zero.
The circuit of FIG. 1 also includes a modulation current source 26 that receives a modulation current set point signal 28, and is coupled to the laser diode 10 via a data switch 30. The modulation current source 26 ensures that when a digital one is needed, there is sufficient current flowing through the laser to produce an optical one. The modulation current source 26 also provides a modulation current monitor signal 32 that is indicative of the level of current in the modulation current source 26. When the data switch 30 is turned on (responsive to a data signal 34), the modulation current source will supply the appropriate amount of current through the diode 10 to product an optical one. The components may be mounted on a single integrated circuit chip 36 as shown.
The Laser Driver is designed to provide the bias and modulation currents required to maintain a steady average power and extinction ratio (ER) over changes in temperature and time. There are two complications, first the characteristics of the laser change dramatically over temperature and second, ER is very difficult to measure.
The output power for the P1 level may be set by measuring the optical output of the laser with a measurement instrument, and then adjusting the modulation current until the correct P1 level is achieved. The efficiency of the laser in mA per Watt (known as the LI), is a measure of how much current is required to produce a particular power level. The laser efficiency changes with temperature and causes the LI to decrease with temperature. Thus as the temperature increases and the LI decreases, the laser will produce less power and the P1 level will not be maintained. The ER for a laser driver and laser combination are typically calibrated by using an external measurement of optical amplitude by an instrument. Additional control systems may be added to increase the modulation current as the temperature increases in order to maintain a constant ER or OMA over temperature. This may be a desirable feature for an optical communications system.
Laser efficiency is expressed as the amount of current required to produce a given output optical power. The ER is difficult to measure because the threshold current at which point the laser begins to emit light varies with temperature. As the efficiencies and threshold change over temperature, the level of modulation current must also be adjusted over temperature if the ER and average power are to be maintained. Certain conventional laser drivers employ look-up tables for different temperatures. A look-up table is compiled for each laser driver based on tests that are run over a range of temperatures. The use of the look-up table facilitates compensating for large temperature variations that occur in the efficiency of the laser over temperature. As each laser diode ages, however, the performance may change. If the look-up tables are not recalibrated, the performance of the system may be compromised. This approach is expensive as it may require calibration of each laser over temperature in order to achieve high accuracy.
It is possible to attempt to measure the LI curve of the laser over temperature, but it is difficult. There is no easy way to measure the optical power to produce a single one bit or a single zero bit as the modulation and switching occurs too quickly: such a measurement system might have to work in the gigahertz range. Many conventional laser drivers perform an average power measurement using a monitor photodiode. The monitor photodiode may receive a small amount of light that escapes from a non-output surface of the resonant cavity of the laser. If the bandwidth of the photodiode is very low, the photodiode averages the laser output very quickly and provides a measurement of average power, not of the peak power. Since ER is the ratio (P1−P0)/P0 and average power is (P1−P0)/2, there are three variable and two unknowns. It is not possible, therefore, to determine either the P1 or P0 level with simply a measure of average power.
Despite the difficulty outlined above, certain conventional laser control systems employ dual loop control as disclosed, for example in U.S. Pat. No. 6,414,974, which permits the measurement of the efficiency of the laser in real time. Such systems may continuously send a small low frequency test pulse that is represented as a small change in power. The systems change the power a small amount that is not enough to disturb the digital signal data. A small fraction of the power is received through a back end facet of the laser diode by a photodiode, and the monitoring of this power permits the extinction ratio to be controlled since the efficiency of the laser (the LI curve) is constantly monitored. Knowing the efficiency of the laser, permits the laser driver to better control the laser efficiency over a range of temperatures.
FIG. 2 shows another prior art laser driver on an integrated circuit chip 38 that includes an output laser diode 40 that provides a laser output signal 42 as well as optical feedback 44 to a back-facet photo monitor diode 46. The photo monitor diode 46 is coupled to Vcc as shown and to an automatic power control loop (APC) 48. The APC 48 measures the current from the diode and compares it to the average power set point signal 52. If there is a delta between those points, the APC 48 changes the bias current at the bias current source 50, which changes the draw of current through the laser diode 40. The system also includes a bias current monitor signal 54 that is indicative of the amount of current in the current source 50 as discussed above.
The circuit of FIG. 2 also includes a laser LI measurement and control unit 56 that receives an optical modulation amplitude (OMA) extinction ratio set point signal 58, and is coupled to and controls a modulation current source 60. The modulation current source 60 is coupled to the laser diode 40 via a data switch 62. The laser LI measurement and control unit also controls an LI measurement test current signal source 64 that is coupled to the laser diode 40, as well as an APC signal from the APC 48. The modulation current source 60 also provides a modulation current monitor signal 66 that is indicative of the level of current in the modulation current source 60. When the data switch 62 is turned on (responsive to a data signal 68), the modulation current source will sink the appropriate amount of current through the diode 40 to product an optical one. These circuits may include either direct connections, or an AC coupled connections where there are capacitors between the laser and the other elements.
The system of FIG. 2 estimates the LI curve and automatically control the value of the modulation current. The automatic power control loop is essentially the same as in the system of FIG. 1. The laser measurement and control block receives the set point as an input. The input, however, is a desired extinction ratio instead of a modulation value. Instead of setting a current therefore, a measure of performance of the laser is being set. The system of FIG. 2 includes a test current source (the LI measurement test current) that is a small current that is drawn from the laser that is controlled by the measurement block. At various times, the LI measurement control block looks at what the automatic power control loop is seeing. When small test currents in are introduced into the system, it is expected that a small change in the average power will result. The laser measurement control unit also has a control signal that is provided to the modulation current source.
Such a system has the advantage of only requiring calibration of the average power and ER at one temperature, and then the system is able to control these parameters over temperature by constantly measuring efficiency changes in the laser. The accuracy of the system is set by how accurately the LI curve may be measured. Existing implementations make an assumption that the laser LI curve is linear over the desired range of operation, i.e., that each additional mA of current added to the laser will result in the same additional increase in laser optical power. Various schemes may also accommodate a simple non-linear component such as a first order non-linearity. For more complex non-linearities, the system fails as it is unable to make an accurate measurement of the laser LI curve, and as a result the ER varies from the desired result. Laser non-linearity tends to occur at high temperatures or at high current levels. Laser efficiency decreases as temperatures and currents increase until the point that further increases in current no longer result in an increase in output power.
Further prior art devices or methods provide that the extinction ratio may be controlled or stabilized using the modulation and bias currents. For example, U.S. Pat. No. 6,807,209 discloses a system for controlling the extinction ratio for an optical transmitter wherein a measurement the bias current used to determine a value for the modulation current in order to control the extinction ratio. This U.S. Pat. No. 6,807,209 discloses that the laser's performance may be improved as temperature increases. Such a system is a method for controlling extinction ratio however, and it does not try to correct for non-linearity.
There is a need, therefore, for a more efficient and economical system for providing linear output power in a laser driver over a wide range of temperatures.